Anodic decontamination of zirconium and hafnium



United States This invention relates to electrolytic decontamination of zirconium and hafnium and their base alloys and providing' the decontaminated metal with a protective coating.

Zirconium and hafnium have a strong affinity for oxygen and since these metals are in common usage in industrial equipment where it is essential that the elements fabricated of these metals or base alloys of these metals be uncontaminated it is necessary to be able to remove contaminants from the surface of structures that are fabricated of these metals or their base alloys and to provide the metal surface with a protective coating. The present practice is to gouge surface contaminants, such as copper, from the surface of these metals and to subject the components fabricated from these metals to extremely rigorous tests to insure that no surface contaminants are present. One of the most prevalent surface contaminants of components fabricated of these metals is copper which is picked up during the welding operation where copper backing bars or the like are employed.

The present invention has for its object an improved system for removing contaminants from the surface of zirconium and hafnium and base alloys thereof and to provide a protective film or coating on the surface of the cleaned metal. 7

The invention involves making the contaminated metal the anode of an electrolytic cell which has an electrolyte of a C.P. water solution of sulfuric acid. The cathode of this cell may be any material which is ordinarily not attacked by the acid. Voltage is applied across the electrodes of the cell at a value such as to maintain a relatively low current density, below 1 ampere per square millimeter of the surface that is being cleaned. The current density is such that only the metallic contaminants on this metal which prevent the forming of a protective oxide film will be attacked with the remainder of the anodic surface having formed thereon a protective coating, which is an oxide coating and the current density must be maintained below 1 ampere per square millimeter during this time with the lower the current density the greater the time required to remove the contaminant and produce a protective oxide coating. This current density which attacks only these metal contaminants, such as copper, is maintained until the metal contaminants is completely dissolved. This leaves a spongy lattice of uncontaminated metal whether it be zirconium, hafnium or their base alloys of these metals, in the zone where the contaminant was originally located. Thereafter the current density is raised to approximately 1 ampere per square millimeter or, in other words, to a value slightly above the value where arcing occurs. Arcing is produced, with the arcing resulting in the spongy metal combining violently with the oxygen liberated by electrolytic action. Any finely dixided material or thin sharp projections will be attacked and a smooth, clean surface will be produced as a result of this arcing. It should be noted that current densities substantially in excess of those necessary to achieve the arcing may cause damage to the metal being cleaned and accordingly should be avoided.

After thus cleaning the metal and removing the spongy lattice the current density is again reduced to a low level in order that a protective film or coating will form on atent 9 "ice the previously contaminated surfaces which are now smooth and clean. The current density is maintained at this low level a suflicient time so as to form the desired film with this being evident when the current falls off substantially because of the resistance or nonconductivity of this film. After this operation the power supply of the electrolytic cell may be deeuergized and the cleaned and coated anode removed.

Should there still be contaminants on the metal after it is removed from the cell these will be plainly visible because of discoloration as the contaminants will not be provided with the protective coating that the remainder of the anode will have. Accordingly the process not only will remove contaminants and provide a protective fihn on the metal but also inherently provides an excellent test as to whether surface contaminants are present. A C.P. sulfuric acid solution is employed since impurities therein are detrimental to the operation of cell. The water acid solution must not be over 200 F. and may be anywhere in the range of from freezing temperature to 200 F. with the concentration of the acid being anywhere from 5% to 95%.

The following illustrates an example of the practice of the invention.

Example The apparatus involved was:

(1) Weldment being cleaned and coated was of Zircaloy-Z, having an area of approximately 8 dm. and being contaminated principally with copper. Electrolyte was a 10% CF. sulfuric acid-water solution there being about two liters container in a glass container and having a temperature of 70 F.

The cathode was lead, about 10 dm. in area and the anode and cathode were spaced about 4".

The power supply was 2 k.v.a., continuously variable from 0-200 V. DC. and 0 to 10 amperes.

In the process, the voltage was gradually increased to 70 V. DC. over about 30 see. In about 32 see. the current flow was zero as shown by the ammeter. The voltage was then increased until arcing occurred (about v. DC.) and maintained until arcing ceased (a matter of a few seconds) thereafter the voltage was decreased to a low value and again brought up to 70 V. DC. until current flow ceased. Thereafter the power supply of the electrode cell was deenergized and the Zircaloy anode withdrawn whereupon it was found that the copper contamination was completely removed, that the zirconium had a smooth surface and was provided with a protectively anodic coating.

Similar results are obtained with hafnium as the anode and with the process controlled with respect to voltages and electrolyte in the same manner above set out.

Accordingly, it is seen that with the present invention zirconium and hafnium and their base alloys may be readily and conveniently decontaminated and provided with a protective film.

It is to be understood that the invention is not to be limited except by the scope of the appended claims.

What is claimed is:

1. The method of removing metallic contaminants and providing a protective coating on metals selected from the group consisting of zirconium, hafnium and base a1- loys thereof which comprises electrolyzing said metal as the anode of an electrolytic cell having a water solution of sulfuric acid, with the acid concentration in the electrolyte being in the range of 5% to and with the temperature of the electrolyte being in the range of just above freezing to 200 F., maintaining the current density substantially below 1 a./mm. until said contaminants are dissolved, thereafter increasing said cur which comprises electrolyzing said metal as the anode of an electrolytic cell having a water solution of sulfuric acid, with the acid concentration in the electrolyte being in the range of 5% to 95% and with the temperature of the electrolyte being in the range of just above freezing to 200 F., maintaining the current density substantially below 11 a. /mm. until said contaminants are dissolved leaving a spongy lattice where the contaminants were, thereafter increasing the current density to a point where arcing takes place but not substantially above the point where arcing is initiated and maintaining this current density until the spongy lattice. is removed and a smooth surface produced, thereafter decreasing the current density to a value substantially below the arcing value and maintaining it at this decreased value for a sufiicient time to cause a nonconducting protective film to form on these smoothed portions of the metal.

3 The method of removing surface copper and providing a protective coating on metals and base alloys thereof of the group consisting of zirconium and hafnium which comprises electrolyzing said metal as the anode of an electrolytic cell having a water solution of sulphuric acid, with the acid concentration in the electrolyte being in the range of 5% to and with the temperature of the electrolyte being in the range of just above freezing to 200 F., maintaining the current density substantially below one ampere per sq. mm. until the copper is dissolved leaving a spongy lattice of metal where the copper was, there-after increasing the current density to a point where arcing takes place but not substantially above the point where arcing is initiated and maintaining this current density until the spongy lattice is removed and a smooth metal surface is produced, thereafter decreasing the current density to substantially below the arcing value and maintaining it at this decreased value for a sufficient time to cause a protective film to form on these smoothed portions of the metal.

References Cited in the file of this patent UNITED STATES PATENTS 2,631,115 Fox Mar. 10, 1953 2,835,630 Huddle et a1. May 20, 1958 FOREIGN PATENTS 815,572 Great Britain July 1, 1959 

1. THE METHOD OF REMOVIMG METALLIC CONTAMINANTS AND PROVIDING A PROTECTIVE COATING ON METALS SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM, HAFNIUM AND BASE ALLOYS THEREOF WHICH COMPRISES ELECTROLYZING SAID METAL AS THE ANODE OF AN ELECTROLYTIC CELL HAVING A WATER SOLUTION OF SULFURIC ACID, WITH THE ACID CONCENTRATION IN THE ELECTROLYTE BEING IN THE RANGE OF 5% TO 95% AND WITH THE TEMPERATURE OF THE ELECTROLYTE BEING IN THE RANGE OF JUST ABOVE FREEZING TO 200*F., MAINTAINING THE CURRENT DENSITY SUBSTANTIALLY BELOW 1 A./MM.2 UNTIL SAID CONTAMINANTS ARE DISSOLVED, THEREAFTER INCREASING SAID CURRENT DENSITY ABOUT 1 A./MM.2 SO AS TO CAUSE ARCING AND MAINTAINING THE CURRENT DENSITY FOR A SUFFICIENT TIME TO PRODUCE A SMOOTH SURFACE, THEREAFTER DECREASING CURRENT DENSITY TO SUBSTANTIALLY BELOW SAID 1A./MM.2 FOR A SUFFICIENT TIME TO CAUSE A NONCONDUCTIVE COATING TO BUILD UPON THE METAL FORMERLY COVERED BY THE CONTAMINANT. 